Photographic exposure calculator



April, 1928.

J. R. HEWETT PHOTOGRAPHIC EXPOSURE CALCULATOR 5 Sheetsheet 1 Inventor John R. Hewett,

b/HKRMAQZ- WW His Attorne April 3, 192e..v

1,664,818 J. R. HEWETT PHOTOGRAPHIC EXPOSURE CALCULATOR Filed ign@ 2. 1922 :s shetsfsneet 2 l..

vInventor John R.Hewett,

His Attorneg vApril s, A192s.- 1,664,818

J. R. HEWETT PHOTOGRAPHIC EXPOSURE CALCULATOR Filed June 2. 1922 3 Sheets-Sheet 3 "muon .mmm

IIIII-:E-L-llmll Inventor John R. Hewet' b5 RMMQ His Atorflg- L-g Patented Apr. 3, 1928.

' UNITED STATES; PATENT OFFICE.

l j J'QHN R. HEWETT, 0F SCHENECTADY, NEW YORK.

. f PHOTOGRAPHIC EXPOSURE CALCULATOR.

Application med June 2, i922. serial No.`e5,4sa.

My invention `/relates to photographic eX- posurc calculators of the type 1wherein .the

light value for a'given time and place, :is determined from an exposure table, is cor- 5 related with the sensitiveness .of the film or plate being used and4 with the subject heilig photographed in order to determine proper rameia eigosure adjustments.

The primary o l0 the provision 'of a simple exposure calcula ,tor which combines the correlating mechanism and all necessary exposure tables in a single instrument. d

A further object is the `provision of anV a minimum the liability of incorrectly ,readv ing the calculator. 4

A further object is the provisionp `such a .3o calculator'wherein only one adjustment is j required to indicate in terms' of Ythe usual camera markings the correct vcamera adjust-r ments for a given plate speed andfor any given subject and liglit value. :sa A further'obje'st of my invention is the provision ot an exposure calculator which g may be converted into an exposure meter by substituting for its tables means for meas# uring the light value. Further objects and jo advantages of my invention will appear as y the description proceeds.

ln my United States Patent'1,575,3381 have described and broadly claimed certain features of the calculating mechanism'described in the present application. In said patent the calculating mechanism is des ribed in combination with means for measof light on a photosensitivc film. My im- :fo proved calculating instrument 1s referably mounted. about the lens tube oft e kcamera.

in fixed relation/thereto and contains all ofy the necessary tables for determining light ject of my invention is l tion on line 3-3 of L #tion taken on line 1111 of Fig.l 11; 1 uring the light value by the chemical action* factors for any time andl location. The light factor v`table for the proper time of day, month, latitude and subject lighting condition will appear in an opening in the instrument. so that all it is necessary Jfor the 7photographer to do is to hold his camera in one hand and' make one simple adjustment oty the calculator with the other hand to determine his camera exposure instructions and then without changing the position of the camera set his camera stop and exposureY pointers to the determined instructions.

rIvhe :features of my invention which I believe to be novel and patentable will be pointed out in the claims appended hereto.y

The details of construction .and the manuel` of using my improved exposure calculating instrument will now be explained in connection with the accompanying drawings in which Fig. 1 is a front view of one orm of mv improved ex is adjustable for di erent plate speeds; Fig.

osure calculator which 1 shows a detail of Fig. 1; Fig. 2 is a sec- 1;Fi .Bisasec- `g. 1 ig. 4 is an enlarged view of a portion of the calculator p12, Fig. 1 withrparts broken away to show tion on line 2-2 of 1 einteriorarrangement; Fig. 5 shows one i side of one of the disc tables used in my i1n-v proved calculator, the tables illustratedI being? for latitude 4:0 deg. NW' Figs. 6 and 7 show opposite sides of certain portions of the calculator for the uniform and `focal systems of camera markings; Figs. 8 and 9 illustrate details of construction of the instrument which permit the ring shown in Figs. 6 and 7 to be reversed, Fig. 9 illustrating a section taken on line 9-9 of Fig. 8; Fig. 10 shows a. front view of ther calculator arranged to liave certain of its scales serve as the4 stop and .shutter timer adjusting scales of a.

camera; Fig. 11` shows a front viewcf s..

strument when lit is desired to change a light tablediscfor' another of a different latitude; Figs. 14 to 17 inclusive showext`ra illustrative4 of its adaptability parts Whichare interchangeable with parts seconds,

of the calculator should it be desired to use the instrument as an exposure meter.

ln the drawings like parts are desi nated by the same reference numerals; Re erri'ng now more in particular to Figs. 1 to 5 inclusive, the body portion of the instrument comT prises a casing preferably in a form adaptedv to tit about the lens tube of acamera and to be secured thereto in any convenient manner so as not to interfere with the proper operation thereof. lt is convenient to malte the casing in an annular form comprising a rear channel section l and a front section 2 having annular outer rim 3 adapted to ht about and be 'detachably secured to the rear section. l in any convenient manner suchas by a plurality ci spaced spring lugs d on rim 3 iittincu into corresponding recesses 5 on the outer channel portion oi the-rear section l (see -Figs. 2 and i3). The inner edge oil front section 2 of the casing does not extend factor. For the purposes of exterior oight ' undcrsimi arconditions. The light inten- Afor obtaining correct 'to the inner channel wall 6 of the back section but .is spaced therefrom so as to allow the insertion ot a relatively movable ring 7 having mar 'nal anges 8 and 8 at its ends. Within the casing thus formed there is provided one or more thin discs 9 carrying tables of light factors for suitable divisions of the ear and day and a spacer disc 10. @ne similia of a complete disc 9 is shown in Fig. 5 illustrating the referred arrangement ofthe light factor ta les. Before proceedin iurther with the explanation of the calcu ating instrument, it will be desirable to explain what is meant by the term li ht tography, the determination of natural intensities depends primarily upon four conditions, namely; the latitude, the time ed year, the time of day, and the'. atmospheric condition. Thus, the light intensity at the equator is ater than at 40 deg; N. latitude sity at d() deg. N. latitude is greater in the summer than ein the )winteig at noon than it is in the morning or evening, and on a clear day than it ist on acloudv day. The relative light intensities for diterent latitudes `and for diierent months, times of day and weather` conditions may be and have been predetermined witha suiiicient degree of accuracy to serve as a uide for determining the propericamera ailiustments to be used photographic exposures. These relative light intensity values may be calculated from the physicalconstents of the universe or obtained :from actual measurement. .Light values have been 'determincd and tabulated by various authors in various ways and therefore it will not be necessary to explain or advocate any particular method. It is desirable however from on-fA v aspect of my ,invention to use a light value unit expressed in units vof time, preferably necesita which unit is the time required to color a standard photosensitive paper'to a standard tint under any particular light intensity.` It requires approximately one sepond to color a certain standardphotosensitive paper to, a' standard tint at the equator on a brilliant day when the sun is at the zenith. f

paper to the standard tint and three secondsbetween the hours of 8:0 0 and 9:00 in the morning and 3:00 and 4:00 in the after.-

noon. For the sake of simplicity thesevalues are expressed in my tables as 2 and 3 respectively.' 4rlhese values and likewise the values for other conditions as expressed ou the tables used in my instrument are hereinafter leferred to as light factors. Thus, l use the term light factor7 as broadly desighating the light value under one condition as compared to the light value under a different condition and I do not wish to be limited by lil the use of this expression to the particular system of units described fherein.

'lihe` light factor tabular disc for 40 deg. hl. latitude is illustratedin- Fig. 5 and is suliiciently accurate for all latitudes between 35 deg. and 45 deg. N. and is marked accordiugly. Preferably the various countries, States and provinces falling within `theselatitudes willl also be included on the disc in the space not required for the light factor. Thus, on the Iinner portion 11 of theV disc shown in Fig. 5, I have indicated the various countries'or divisions thereof for which this particular disc may be used. On the outer portion 12 of the disc'there are arloo Iln

1anged12light factor tables indicated by the 4 portionsl, one for each month of the year.` The name of the month4 for each table is preferably indicated'at the bottom thereof and the tables are arranged in. consecutive order as illustrated. The day division limits for4 the light fdctors are indicated4 at the topofeach table at 14;. The significance of this arrangement ,will best be understood by referring noixpfto Figs. 1 and 4 where it will be noticed that opposite the outer portion lao i.

12 of the tabular discs there is provided an.

opening 15 in the front cover 2 of the casing which opening occuples a sector 1n this mstancc of exactly 30 degrees and extends to the outer and linner edges of that portion 12 of the tabular discs 9 occupied by the monthly tables. It will thus be obvious that any one of the light factor tables appearing on the f opposite the names front of the uppermost dise 9 together with the corresponding day division limits and namefof the month may -vbe iliade to appear in this opening. In Figs. 1 and 4 the monthly table for June ofthe disc for 40 deg. N. latitude appears in the opening 15. In Orderfto facilitate ythe moving and correct positioning of the upper disc 9 so as to bring any-,desired monthly table exactly opposite opening 15, I have provided each discwith small holes 16 near the outer c'ircuii'ifereiice of the' months and a cor-` responding opening 1G in the upper edge of opening 15 sc th'at when any one of the monthly tables of the upper disc 9 is correctly adjusted with respect to opening 15, oneof theholes 16 will coincide-with yopening 16. Now all that is necessary to do tol advance, the disc one month is to insert a opening 15. exactly degrees and pencil point, pin or siiiiilareobject into the coinciding openings 16":ind 16 and move the upper disc downward until thev pencil or-v other'object abuts against the lower edge of Thisadvances the upper disc brings the next following monthly table into correct alineinent with opening 15` and'the nextliole 16 into alinement with opening 16. I have found vthat the ordinary nietal stylus used with autograph cameras serves the purpose for moving the discs very well. The total thickness of the various discs 9 and the spacer\ Vring 10 should be justrsutlicigent to allow the upper disc to vbe moved in' this manner with slight friction.

On the outer circumference of cov-er 2 oppositegopening 15 there is provided a sector plate 17. This plate is secured to the 'cover' 2 in any suitable manner and is preferably set intotlie'cover so that their outer surfacesare flush with each other. If desired, this plate lmay be ,stamped integral with cover section 2. The plate 17 is divided by radial lines into seven sections 'which correspond to the seven radial sections on each .monthly table ofrthe discs 9. The seven sections `of plate 17 reading-fron top to bottom are lettered as follows: Day ivision'limits Brilliant Bright Ditused v Dull Very dull'and Month. The live intermediateisections refer to the light on the subject bein ""photographed as indicated in the margin of that portion of the plate 17. It will nowbe seen from an inspection of Figs.

.lier 4, that when the upper disc 9 is corv.rectly adjusted respect rto opening 15 for any given month, sectionslll, 13 and 16 of dise 9 will appearopposite the sections of sector plate 17 marked Day division limits, Subject lighting-V and Month respectivelyf .The yportions 13 and 14 of the monthly tables ,on discs' 9 are divided into y y threegroups'by circumferential lines for the v,purpose of indicating theproper day divi- ,with heavy refer. Thus, referring to Fig. 4. it `will be j,

seen' that between the hours of 8: 00 and 9 00 iii the morning and 3:00 and L1:00 iii the afternoon for latitude 40 deg. N. in the month of June. the light factors contained inthe outer column should be used; at inidday which for this table is from 9:00 a. In.

to 3:00 p. m., the middle column of light Opposite opening 15 oii cover 2 the factors should be used; and for early morn- I ing between the hours of 7: 00 and 8: 00 and foi' evening. between the hours of 4: 00 and 5 z 00,'the inner column of light factors should be used. In order to facilitate the reading of the instrument, I prefer to arrange the day division limit figures the same side up as the corresponding llettering on the portion 18 of the cover and at an angle of 90 degrees from the upright position of the light. factor values and the corresponding lettering on plate 17. It will be seen from an inspection of Fig. 5 that the day division limits and light factors lare not the saine for all months but vary in accordance with -actual conditions which have been predetermined in any desired manner.4 Certain ot is shining on the'subject unobscured by cloud. mist or shade, the light factor would be selected from the radial iow marked Brilliant; if the sim is covered by light clouds or a slight mist or smoke, but the sun still throws strong shadows 4lou` the subject, the light factor would be selected from the radial row marked Bright., If there is a general even wheie cast shadows are just distinguishable,

dn1t,no direct sunlight, the row markedv Difused should be used. lVhen the is covered wLitli dulll clouds or the subject is fairly well shaded and notast shadows are distinguishable, the row marked Dull should be used,

gloomy clouds, the subject being in the open or' where the sun is bright, but the subject is heavily shadedfi as ina thick woods,`the row marked Very dull should be used. It will be of considerable assistance and when the sky is overcast lightn on` the subject such .as

in determining the lighting of the Subject to look at the same throughv the view-finder.

` Of course, it wouldbe possible'to use anydegree of refinement inl selecting the divi- -'sions for the latitude, year, day and subject JH n with Y my instruuunt 'for all inhabitablc countries of the world maybe combined into eight disc shaped tablescalculated foithe following latitudes, the fractional form giv en after each latitude indicating that the table is suitable for use'over that range,

-llachy of the four vdises Slshown in Fig. 5 has one'such table on either side so that only the four discs a re requiredior a worldvidc instrument. lf desired. a disc might be provided for eachlatitudc division in which case the names of the various countries for thegiven latitude might be (lll contained on the opposite side of the disc from the table leaving more room for the light factors which could then bc spread over a greater radial depth of the disc and the opening enlarged accordingly. ,A Preterably, the discs 9 are made very thin so .as to be slightly flexible. They may be made from any suitable material such as celluloi d o1;l thin sheet brass or other metal. The tables may be etched, engraved, printed or carved on orin the disc in any suitable manner.' The spaJcer ring 10 will he just thick enough to 4producea snug tit and still per-f mit the upper ring to be easily moved.

Ring 10 will preferably/.have a larger inner diameter than the dises` 9 so vas to allow. plenty of room` for the inner flange 8 oit' ring 7. lt will seldom be necessary for the ordinary photographer, unless he travels, to change the discs for latitude, butdf it is desired to do so, the case may be 4openwed by placingan instrument such as the point of a knite in the opening T19 shown in Fig. 13 to pry it open as illustratedv in this figure. Then the proper disc and table will be placed uppermost and the easing closed. The -disc "must also be advanced once a month as previously described.

I will now explain the apparatus for and manner of using the light factors in determining camera exposure adjustments. On the front of cover 2 there 1s arranged al scale designated in general by the numeral 20, which in Fig. 1 is graduated Afrom 64 topi/10() seconds in a sufiicientV number of steps to accurately take care of all light conditions and exposures4 liable to be met with in ordinary photographyI It will be` noted that below 1/10 this scale 1s not marked in the fractional form, but is marked and'.100 indicatin'v'1/25, 1/50 and 1/100, seconds respectivcy and is thus marked so as to correspond with the form of exposure time markings commonlyiound on cameras'. Each graduation of this scale is also marked 'l".B or Snap corresponding to the usualJ camera exposure time markings, indicating respectively 'lnne exposure," Bulb exposure and Snap shot. 'lhe sigmhcancc of thesemarkings will be clear when the coniplete operation of the instrument is hereinafter described. ln practice it will be found that the part oi' scale 20 trom 1 to (54 will be the only portion used lfor light factors as the lowest. light factor unit which l use isl l. The entire scale 20 will be used for exposures. 'llhe respective ends oli' the scales may be marked accordingly ina manner suggested in Fig. 1 1. Coopc 'ating with scale 20 is a subject scale 2l and a Stop scale 22 respective-ly mounted on an ovcrhanging sector 3 ot' flange 8 and a ring 24. These parts are held together and: to .the front cover 2 hy means otl the anargina-l flanges 8 and8 on ring 7 as clearly shown in Figs. 2 and 3. 'lhe rings which carry the subject and stop scales are rotatably adjustable with respect to the scale 20 contained ou the i'ront cover 2 and in Fig. 1 the stop -and subject-scale rings are ladjustable with respect to each other for the purpose of making the instrument adjustable for, (liderent plate speeds. The ring 24 as it appears when removed fromthe instrument is shown in F ig. 6 Where it willbe seen that a plate speed scale 25 is contained on vits inner edge. This plate speed scale is covered up by the marginal' flange 8 of ring 7 when 4the parts are assembled las in Fig. 1, except at an o'peninoi 26 therein. The plate speed and stop sca es are so related With respect to this opening that when the plate speed 'lim beingI used appears 1n opening 26, the instrument is correctly adjustedfor this particular plate speed. Thus, in Fig. 1, the instrument is adjusted for a plate speed of 512. In o'rder to make the ring 24. adjustable with respect to" ring 7 and still permit them to be moved together with respect to scale 2() as a. single dial, means are provided.

to normally hohl rings 7 and 24 in fixed relation.'I To thisend ring 24 is provided with a plurality of spaced notches 27 4in its outer edge` which cooperate With a finger 28 formed by .a turned l down port-ion of one end of sector 23. A handle 29 is formed by a turned up portion of the same end adjacent finger 28. This is .best illust-rated in Fig. 1", which shows an end View of sector 23. The finger 28 ts Iinto the notches-27 and normally prevents the relative movement of sector `stop of the two rin s. When -it is desired lto adjust for a di erent plate speed, the end 23 together with linger 28 is sprung away from ring 24 a suicient'distance for the linger to clear the notches 27 at which time the two rings may be adjusted relative to each otheruntil the desired plate speed appears in opening v26 at which time lthel upward pressure Lon sector 23 is released and finger 28 is allowed to move into a different notch 27, the notches beingy correctly positioned for this purpose. landle 29 is provided to facilitate` the adjustment of scale 21 with respect to scale 20. It will be under` stood that the various scales have been care# fully compiled and are so associated with each other in the improved calculator as to give accurate results when correlated in the manner now to be explained.

The operation of thel instrument is best illustrated bymeans otl an example. us'suppose the photographer wishes to take some pictures in Vashington, D. C. .in the inontlt of June. Since Washington, D. C. lies within the latitudes of 45 and 35 de N. the disc table for 40 deg. N. latitu e will be placed uppermost in the instrumentl and will be adjusted until J une appears opposite nionth on sector 17. Let us suppose the camera Lis loaded with a film having a plate speed of 512. He therefore, adjusts the ring 24 relative to ring- 7 until 512 appears in opening 26. At 8:15 a. m. on a bright day he desires to photograph a normal subject in the open. From the exposure "'.able hc notes that he should use the -factor 4. He then sets Normal on the subject scale opposite 4 on scale 20. The correct exposures for each camera stop is'then indicated opposite stop scale 22.'4 The adjust- .ments above referred to, are indicated in Fig. 1 where it will be noted that if a snapv shot is desiredeither stop 4 or 8 may be used, rgivingthe exposure 1/25 of a second vwhich is 'indicated as at 25 in order to correspond to the ,usual camera markings. A bulb exposure may also be made using any one ofthe combinations indicated thus 16 and 1/8 of a second exposure, stop 32 and 1/4 of 'fav second exposure or stop 64 and 1/2 of a second exposure. The camera stop and exposure pointers arethen set to the selected combination and the picture taken.' Further examples will be given in connection with other modifications.

The calculator as thus far described may be carried in the pocket if desired. =It is generally preferable however to mount the instrument in lfixed relation to the camera in a position to be easil inspected when setting `the camera sto an shutter timer pointers and t this en 'the instrument is made in the annular form illustrated, of the proper size to fit about the lens tube of a camera in a manner illustrated in Fig. 12 and secured ylator shown in theretotin any suitable manner. The usual stop and shutter timer scale camera indicated at 30 and 31 respectivel in the various iguresare usuall fastenedE to the camera by screws 32 indic ted in dotted lines in Fig. 12. I have found it convenient to fasten my instrument to the camera by providing screw holes 33 (see Fig. 4) in the back portion 1 of the casmg, positioned to come in alinement withthe usual screw holes used in fastening the plates 30 and 31 to the camera 'and passing the screws 32, or if necessary, screws slightly longer, through the back section l of my instrument casing clamping the plates 30 and 31 between it and the camera. The plates 30 and 31 may also be secured to the back l of the casin by hollow rivets or screws positioned in anement with the usual screw holes in the camera. The calculator might also be secured about the the lens tube by means of a rubber gasket compressed between the outer surface of the lens `tube and the inner surface of the calculator casing. I do notwish to be limited to any particularimethod of fastening the instrument to a camera. If desired the camera wall of thejinstrument casing may be extended as indicated in dotted lines in Fig. 3 to form a seat for the usual cap or the color screen.

There are two different systems of platedesignated thus U. S. Another system commonly used. with astigmatic lenses is called the focal system and is designated by the letter F. The scales of the calcu- Figs. l and 10 are designed for the uniform system and the calculator shown in Fig. 11 is designed for the focal system. In view of the two different stems of markings above referred to it elates of a comes important to provide a calculator f which may be readily ada ted for either system. Figs. ,6, 7, 8 and 9 1 ustrate a co'nvenient way of accomplishing this object with one form of m calculator without increasing thenumber o lnecessar parts. Figs. 6 an 7 represent opposite si es of the ring 24 and the stop scale late 30. The 'ring 24 and plate 30have eir opposite sides provided with scales designedl for the two systems, Fig. 6 illustrating the uniform scale system and Fig. 7 the ffy/,al scale system. It will now be evident that a calculator provided with such parts may be ada ted for either the uniform or focal systemy merely reversing these parts. Thus, the calculator shown in Fig.` 1 could be changed to a focal system calculator by merely reversing rin 24 andv plate 30, provided the reversed sides of these parte are marked with. the scales' illustrated in be reversed in a convenient manner the ring ring 7 of Fig. 8. Here I have illppstrated the ring 7 as being split at 34 in aI manner to i' permit the end 34to be sprun inward, as

indicated in dotted'lins, a suicient distance yto release ringA 24 which maybe then removed, jturned over and inserted again. This must be done when the cover is removed as in Fig. 13 because the inner wall 6 of the back section-would otherwise prevent1 the end df from being sprung inward, as will be clear from an inspection of Eigs. 2 or showin the parts assembled. f In Fig. l have illustrated my improved calculator with portions of the outer scale 26 being utilized to serve as the setting scales for the stop and shutter timer pointers respectively dit a camera. The-shutter timer sca e shown on this calculator is designed to be used in connection with the more expenvle sive high speedv cameras. rlhus, the snap shot f or instantaneous exposure scale shown at the top of the meter is graduatedvin small' steps from one second down to 1/300 of a second.

Except for the rearrangement in scale y2O,

the purpose of which will beexplained later, the instrument is otherwise similar to thef calculator shown in Fig. 1.k rllhe shutter timer and stop scale plates however are omitted because, as previously mentioned, portions ofthe outer scale 20 of' the calculator take their-places. Y Assuming the instrulment to be mounted about the lens tube of a camera in the manner previously described,

35, which will be connected tothe mechanism for adjusting the iris diaphragm, and thev shutter timer pointer, designated at 36, which will be connected to the shutter timer adjusting mechanism, are respectively mounted adjacent the lower and u per portions yoi' scale 20'., l have not thong t it necessary to illustrate the connections between thejpointers 35 and36 and their res ective mechanisms on the 'camera as it wil beevident to v those fami-liar with cameras that the proper motion transmitting cams may be provided to make the adjustments ot the camera correspond to the scale indications on the calculator.

Let usi ,Washingtom D. C. in d' une desires to photograph a distant view at d: 00 p. m. with this vtype of calculator. His camera is loaded with ya film having a plate s eed of 768. The'yplate speed scale is there ore adjusted until 768 appears in opening 26. rlhis `adjustment is \one notch to the right from theadjilstment shown in Fig. l and this adjustmentf moves the sto scale a correspondingv amount. He in s the` sun to be covered with dull/clouds and no cast shadows are distin ishable; Referring to the light factor tab e he notes that at 4 :00 p. m. with 0,5 a dull light on the subject he vshould use'byfdotted lines,

the stop pointer of the/camera designated at` assume that the photographei1^ in neonata l factor 12. He' therefore moves the subject scale until Distant view coincides with 12.on scale 20. Stop pointer 35 is then set opposite the number on the lower Apart of scale 20 which corresponds to the number 7o of the stop it is desired to use, for example,

stop 16 and shutter timer pointer 3 6 is moved. opposite this number onstop scale r22, which setspointer 36 for an Iinstantaneous exposure of 1/10 of a second.l These adjustments are 75 illustrated in Fig. 1-0 and 'the camera is properly adjusted to snap for' these conditions. The same adjustments might be used to photograph a normal subject where the li ht factor is d. Likewise, these calculator S0- ,adjustments might be used lto take a snapvv shot of a dark .object near the camera with Va light factor of 2 using stop 4 .in which fcase pointer 35 would be moved opposite@ and pointer 36,would be moved opposite r/eo. f f e When the light is oor or the plate speed of the film o1" plate eing used is ot a comaratively low .i/alue', it4 will sometimes be (esirable to use a time or bulb exposure, @u particularly if the subject being photographed is of such a nature as to require a comparatively` large amount of light. For example, let us assume the following conditions: llate speed 768, light factor 2d 95 and subject Dark object near. ln this case the subject and light factor scales should be adjusted to make dark object near appear opposite 24. This adjustment will bring -sto numbers 32' and 6d on scale 22 1"" opposite hght factor numbers 2 and l respectiue/ly on scale 20' indicating that it either oe of these stops are used, the exposureV should vbe a time or bulb exposure. Thus, tor example, shutter timer pointer 36' 105 will he set .opposite T on the 4upper right of scale 20 and stop pointer 35 will be set opposite 32 on the lower right of* scale 26 and lthe exposure given two seconds. ln

order to assist the photographer in deter- H0 mining whether he should use an Iinstantaneous exposure or estime exposure under any given conditions, ll have divided the scale into two parts, one markedxinstantaneous exposures and the other marked time and bu b exposures, the latter marking being covered u in Figr 10 by the subject scalef 21 the en s of the reference arrows therefor being shown at either end of the subject scale, indicating the limits. 120

lnFig. l1 'I have illustrated a modifica- V tion of my improved'calculator which will berefperred to as the` shrouded type.. The

backsection 1 of the casing, the-light factor discs`9 and ,spacer discs 10 are exactlysirni- 125.

l turned over and under the inner edge ofcontained on a front plate 37 which is adjustable with respect to theI cover 2 and is secured thereto by having its inner edge cover 2 as indicated at 88. The outer edge of plate 37 is turned over slightly as indiing 15.

cated at 39 for the purpose of slightly spaciing it from cover 2. 'lhe cxtreme outer edge'of plate 37 is preferably provided with a milled surface in order that the same may lbe gripped and easily adjusted with respect to the'cover 2. A handle might also be provided as at 29 in' Figs. 1 and 10, if desired. T he calculator of Fig. ll is not adjustable for different plate speeds; that is to say, it is a iixed plate speedcalculator which in this instance is for 111 using the focal system. The plate speed scale 22may therefore be, mounted directlyon the plate 3T which carries the subject scale 21 and a separate rimr for that purpose, such as shown in Fig. 7, is unnecessary. rlhe calculators of Figs. l and 10 might lalso be designed for a fixed ,plate speed, if desired, in which case the ring 24 would be omitted and the stop scale Would be carried directly on flange 8 extended.l Adjacent the names of the day divisions on plate 37 of Fig. 1l a radial opening 15 is provided coinciding as tolength with the width of opening 15 in cover 2. The Width of opening 15 is just sufficient to read the day division limit values on the upper tabular disc 9 when plate 37 1s moved so as to bring open1ng15 over sald values. A Preferably a pointer 40 is pro-5v vided integral with plate 37 extending from opening 15 slightly over the indications on sector 17 so that when any one of the month-- ly tables on dise 9 is correctly adjusted with respect to opening 15 and plate 37 is moved to bring pointer 40 over an indication-on f' sector 17, the values on disc 9, corresponding with such indication, will appear in bpen- An opening 41 isfv provided in plate 37 adjacent the subject scale 21 in order that the light factor values on scale 20 adjacent the subject scale may@ be seen. Likewise,

openings 42 are provided in plate 37 opposite each stop number in order'that the exposure mstructldns on scale 2() appearin oppositesaid stop numbers may be rea The various openings may be marked onJ plate 37v as illustrated to indicate the nature 'of the values Whichshould appear4 therein.

Likewise, the stop scale is labeled thus: F. stops. Preferably the plate speed for which the instrument is designed is also stamped thereon. The exposure timer scale plate 31 and the stop scale plate 30 will be similar-to those previously described, the stop plate bein marked forJ the-focal system. In the s rpuded type of caleilat'or shown in' 11 all portions of the disk 9 and scale 20 not susceptible of use for a given adj ustment are covered up by. plate 37 -whereby confusion in reading and setting the instrument is prevented and the liability to make mistakes with this type of calculator i-s reduced to a minimum. T he operation of this caldulator 1s in general similar to those .previously described. To adjust the disc 9 l ticular time of day and pointer 40 will then be turned to the proper value of subject lighting and the light, factor noted.` 'l`hen the plate 37 will be turned until the proper subject is opposite the selected light factor and all of the proper combinations of stop and exposure instrudtions will vappear opposite. the stop scale. jFhese adjustments may all be made without removing the hand from the calculator as onlyy small circular movements of plate 3T will be necessary. Pointers 35 and 36 will then be moved to the sethrough into the opening 16 of cover 20 and f lected combination of stop and exposure .time and the picture taken.

All parts-oiI my improved calculator are so designed that they may be stamped from sheet metal and assembled in a very simple manner. This assures a low cost, for a vthe stamping dies are made the calculator parts may be quickly turned out inl large quantities. The parts being made 'of metal, there is nothing to wear outv or become soiled as would be the case with the usual book form exposure calculators. By providing circular tables arranged as a two-dimen sional chart which may be moved' past an opening provided .with the two-dimensional' camera adjustments possible to use for a given set of conditions. These indications are given in terms of the camera markings'r .sol that no calculations or interpolations are necessary. Furthermore, the instrument may be used for any plate speed or for either ot' thecommonly used camera marking systems. Finally, the instrument may be mounted in fixed relation to a camera soas to be operated with one hand and in the most convenient position to be seen when preparingto take a photograph. The instructions for-using are simple and almost 'seltevident from an inspection of the calculator-and the amature photographer may be assured that it he has properly followed these instructions in taking a picture, the exposure is sufficiently accurateto obtain a goodphotograph without resorting to the usual practice ot taking several exposures with the hope of getting one good one.

lin my United StatesPatent 1,575,338, entitled Photographic exposure meter ll have described and claimed an instrument tor measuring light values by means oit a photo- .sensitive paper together with scales arranged in the same units and in muchv the same manneras in the calculator ot my present application; ln the meter of my prior patent a disc of photosensitive paper was contained within the instrument casing in a manner very similar to the manner in which the light factor disc tables of the present caluculator are arranged andmeans were provided for moving sensitive paper past an opening in the meter cover surrounded with a standard tinted field tor the purpose ot exposing a portion ot said paper pto the light and noting the time required'to color the same to the shade oit the standard tint for the purpose oit measuring `atinted.tie'ld for substantially clbsing the opening l5 in cover 2. v These extra parts are illustrated in reduced size in --Figs. 14 to 17 inclusive. Fig. 14 illustrates a split springihub 43-ot the correct diameter to lit within the instrument lcasing With the inner.

diameter of the hub riding on` the inner surface of l the Wall 6 oit the back section l ot the casing. rllhis hub is provided with a bent over finger 44'Whichjis adapted totit linto a corresponding hole in the bach Wall l of the-instrument casing so that the`finger piece Willxtend through the opening lt) provided in the outer wall ofcasing l (see l -ltighliY .The extensions 46 and 47-of hub 43 will,come just inside ofcasin 1 and will extenda smalle distance `oneit 'er side of `opening 19.f Figl illustrates a wheel 48 of the correct dimensions `to fit Within'. the `instrument casing, it having the. same inner and outer diameters as discs 9.' This vwheel Locaal has a plurality of radial teeth 49cut in its outer surface and a plurality of rearwardly bent spring sectors 50 cut in its inner surface. This wheel'is designed to support a disc otrsensiti-ve paper 51 shown in `ig. 16 on its upper surface and in order to provide a'driving connection between the paper and the Wheel one tooth 49 is made long enough so that `the paper disc llmay be passed beneath this tooth and' over the remaining teeth. The extension f4? of 'hub 43 is bent forward so as toform a pawl for engaging `the teeth 49 when the wheel is placed`on hub 43 in assembling the parts. A linger 52 `i'ormed by the slit adjacent opening-19 in the outer wall of casing l is bent inward to :torni a ratchet for 'engaging teeth '49. When these parts are assembled, paper disc 5l will be uppermostrand hub 43 lovvermpst. lFinger 44 will extend into the opening pro* vided for it in the back Wall ot casing l and finger piece 45 through opening 19. When the cover 2 is in place springs 50 will be under slight compression so as to `hold the paper disc snugly against thev inner Wall ot cover 2 and hub43 against the back Wall l. Then, by pressing downwardon nger 'piece 45 the wheel 48 and disc 5l will be advanced the distance ot' one tooth. By r@- leasing linger piece 45 it will come back to said disc oit photo\` its original position due to :its ovvn resiliency and ratchet 22 will prevent the wheel 48 trom being retracted with it. 'lhe width of opening 19 is such as to limit thexmovement` ot finger piece"45 as.indicated above. The parts .thus described form a lsimple means ot advancing the disc ot' sensitive paper by a steplby-step movement in a clockwise di. rection. rlhe plate 53 shovn in Figi. 17 is adapted to lit over opening l5 in cover 2, it having a stepped under surtac'e as indicated in dotted lines so as to permit its central portion to set down in opening l5 with the outer edges extending a small distance over cover 2 andthus prevent any light getting to the sensitive paper 51 except through thesmall opening 54e The plate 53 may be "secured in place in any convenient manner..

When the instrument is used asa meter the subject` lighting sector 17 may be removed. llt Will necessary to assemble the meter parts in a dark room in ordereto protect the sensi- -tive paper: llt wasA previously stated that the lightfactor units of the calculator were basedon the time to cololl a standard photosensitive paper to a, standard tint, Which time was onesecond' at the equator at noon on a brilliant day. The paper and tint to ends, required to coor the photosensitive'y ,paper 51 to the standard tint` 55 will be subbe usedrinthe meter willconform to' these sta'ntially the same as the light factor values vgiven by the tables under slmilar conditions of time, latitudeand subject lighting. Of v moans course`the value obtained by the meter will generally be slightly more accurate-than that given by the tables but if the number of seconds'obtained by the meter is noted and the nearest value thereto on the scale is used as t-he light factor, the resulting error may be disregarded as it will never be suiposure.

cient to spoil the picture. lVhen the instrument is assembled as a lmeter it`s `use may be explained as follows: An unexposed portion of the sensitive paper will be advanced into the opening 54 and there exposed to the general light falling on the subject to be photographed. The time for the sensitive paper to color to the standard tint l55 will be noted. Let us assume, for example, that the time to ycolor is 13 seconds. The nearest light factor uniton the instrument is 12 and consequently the nature of the subject being photographed as indicated by the subject scale will be set opposite 12 and the correct combinations of shutter timer and stop adjustments will be obtained as previously described. Conditions might arise where the timeto color obtained is halfwayl between two of the light factor valuesl on scale 2O in which case either of those values may be used in determining the proper camera adjustments. Let us assume, for example, a time to color of 40 seconds, 'subject-Clouds, plate speed 768, and the selected stop as 1(5. Referring to the instrument of Fig. 10, which has an instantaneous exposure sca e more finely graduated than is ordinaril used, we see that if light factor 48 is use ,t

the shutter timer pointer 36 y should be set at 1/10 while if llght factor 32 is used, the pointer should be set opposite 1/15, the next smaller exposure adjustment given. It will be evident to photographers that a variation of this extent is permissible without noticeably affecting the correctness of the ex- The same considerations arise when using the disc tables of the calculator. For

.l example, at 3:00 p. m. in June with a brilliant subject lighting we could use either llight factor 2 or 3 or if at .12 oclock, it

was undecided weather, the subject lighting was Brilliant or Bright, either factor 2 or 3 could he used. From the above considerations it will be evident that a greater degree of refinement in the graduations of the light factor tables as to the'longitude, day division limits or subject lighting indications would only complicate the instrument without it being possible to obtain more accurate exposures because the exposure instructions obtained by the present tables are as accurate as can be used with the present refinement in camera exposure adjustments.

Itwill be evident that by increasing the outer diameter of the instrument casing, both the disc tables and the photosensitive paper light measuring means could be commed in the same instrument,. one outsidethe other. Such a combination would constitute a universal photographic exposure calculator both for ,exterior and interior photo ,raphy-with the advantages ofboth thel t; les andlight measuring means.

The vlight factor tables of my improved lcalculator have been com uted for sea level and are correct for this a titude. If the instrument is vto be used considerably above sea level, the next lower li ht factor should be used for each 5000 feet edevation. It will rarely be necessary to make this correction. 1n accordance with the provisions of the patent Vstatutes, I haye described theprinci le of operation of myl invention, together with the apparatus which I now consider to represent the best embodiment thereof; but

I desireto have it understood that the apparatus shown is only illustrative and that the rinvention may be carried out by other means.

2. An instrument for use in photography comprising anl annular casing, an annular disc in said casing provided with a plurality of equal sized light factor tables, each arranged" as a two-dimensional chart, an opening in lsaid casing the same size as one of said tables, the two dimensional chart indications for said light factor tables flanking said openingon two sides and means for positioning any one of saidtables in said opening with the pioper light factor values in alinement with their corresponding marginal indications.

3. In combination with a camera; an exposurecalculator provided with light factor tables, said tables being graduated in the same units as the light factor scale of the calculator and the camera exposure instruction scales of said calculator being graduated in terms of the corresponding scales on the camera whereby all mterpolatlons are avoided.

4. An exposure calculator casing provided with an opening, a subject lighting scale adjacent one edge of said opening and a day division limit scale adjacent another edge of said opening, and a plurality of light4 factor tables within said casing the same size as said opening. said tables being so arranged that any desired table may be brought into said opening and correlated with said scales.

In witness whereof I have hereunto set my hand this 31st day of May, 1922 JOHN R. HEWETT. 

